Some display devices for performing an image display operation and an image reading operation have been proposed (see patent documents 1 through 4).
An image input/output device disclosed by patent document 1 includes an image input/output panel, in which a plurality of light emitting element/light receiving element pairs located in a matrix on a plane. The light emitting elements act as display elements. For performing an image display operation, the light emitting elements are driven based on image information. An image reading operation is performed as follows. First, a sheet of paper on which color gradation information of letters or the like is recorded is placed on the image input/output panel, such that the surface having the information faces the image input/output panel. The light emitting elements emit light in this state to irradiate the paper with light. The reflected light is received by the receiving elements. Thus, the image information is obtained.
Patent document 2 discloses a display/input liquid crystal panel, in which a receiving element is incorporated in each of pixels. An image reading operation is performed as follows. A sheet of paper is irradiated with light, and light reflected by the paper or light transmitted through the paper is incident on the display/input liquid crystal panel and received by the receiving elements. Thus, image information is obtained.
Patent document 3 discloses an image reading/display device, in which light receiving elements are incorporated in a liquid crystal panel of a passive matrix system. Image information is obtained in substantially the same manner as the display/input liquid crystal panel disclosed by patent document 2.
When a display device as described above in which receiving elements are incorporated in a liquid crystal panel performs an image reading operation, the liquid crystal cell acts as a light emitting element. Light emitted by backlight or the like is transmitted through the liquid crystal panel to irradiate a document sheet or the like. The obtained reflected light is received by the receiving elements.
Hereinafter, an operation of these conventional display devices will be described.
FIG. 12 schematically shows an image input/output device 500 disclosed by patent document 1. The input/output device 500 includes a rectangular substrate 501 and a plurality of pixel sections 502 located in a matrix of m×n on the substrate 501. FIG. 13 shows the pixel sections 502 in more detail. Each of the pixel sections 502 includes a light emitting element 502a/light receiving element 502b pair. The light emitting element 502a is, for example, a light emitting diode, and the light receiving element 502b is, for example, a phototransistor. The light emitting element 502a and the light receiving element 502b are located adjacent to each other, with a light emitting face of the light emitting element 502a and a light receiving face of the light receiving element 502b being directed upward.
Next, with reference to FIG. 14A and FIG. 14B, an operation of the image input/output device 500 will be described. For an image display operation, the image input/output device 500 performs substantially the same operation as a dot-matrix system display device. As shown in FIG. 14A, the light emitting elements 502a corresponding to dots are each put to an ON state or an OFF state based on image information to display two-dimensional graphics, letters and or like. For an image reading operation, as shown in FIG. 14B, a document sheet 503 having graphics, letters or the like drawn thereon is placed on the image input/output device 500. At this point, the document sheet 503 is placed such that a surface thereof having ink 504 used for drawing the graphics or letters faces the pixel sections 502. The light emitting elements 502a emit light sequentially, and light reflected by the document sheet 503 is received by the light receiving elements 502b. Light reflected on an area of the document surface which has the ink 504 and thus has a relatively lower reflectance is weaker; whereas light reflected on an area of the document surface which has no ink 504 and thus has a relatively higher reflectance is stronger. The intensity of the reflected light is represented with binary data in this manner, and thus image information representing the color gradation pattern on the document sheet 503 is obtained.
The display devices disclosed by patent documents 2 and 3 use liquid crystal cells instead of the light emitting elements, but operate fundamentally in the same manner as above.
FIG. 15 schematically shows a display/input liquid crystal panel 600 disclosed by patent document 2. The display/input liquid crystal panel 600 includes a plurality of pixel sections 650, a plurality of gate lines 606, a plurality of source lines 607, and a plurality of signal lines 608. For the simplicity of explanation, FIG. 15 only shows one of each type of elements. The gate line 606, the source line 607, and the signal line 608 are provided on a substrate (not shown) in a lattice. The source line 607 and the signal line 608 are located alternately and parallel to each other. The gate line 606 are located as crossing the source line 607 and the signal line 608. At an intersection of the gate line 606 and the source line 607, the pixel section 650 is provided.
The pixel section 650 includes a liquid crystal cell 660 and a thin film light sensor 602. The thin film light sensor 602 acts as a light receiving element. The liquid crystal cell 660 includes a thin film transistor 601 as a switching element and a pixel capacitance 605. The gate line 606 is connected to a gate electrode of the thin film transistor 601, and the source line 607 is connected to a source electrode of the thin film transistor 601. The pixel capacitance 605 includes, for example, a liquid crystal capacitance and a storage capacitance (not shown) provided parallel to the liquid crystal capacitance. The liquid crystal capacitance includes, for example, a pixel electrode (not shown), a counter electrode (not shown) facing the pixel electrode, and a liquid crystal layer (not shown) provided between the pixel electrode and the counter electrode. A drain electrode of the thin film transistor 601 is connected to the pixel electrode.
The thin film light sensor 602 is located in the vicinity of an intersection of the gate line 606 and the signal line 608. One of two ends of the thin film light sensor 602 is connected to the gate line 606, and the other end of the thin film light sensor 602 is connected to the signal line 608 via the diode 603.
When a voltage corresponding to image information is applied to the gate line 606 and the source line 607, the thin film transistor 601 is put to an ON state to apply a voltage to the pixel electrode. Thus, an image is displayed. When a prescribed voltage is applied to the gate line 606, an electric current of a value corresponding to the intensity of the light incident on the thin film light sensor 602 flows from the gate line 606 to the signal line 608 via the thin film light sensor 602 and the diode 603. For example, as the light intensity increases, the value of the electric current increases. By detecting the value of the electric current, the intensity of the light received by the thin film light sensor 602 is detected. The diode 603 is provided for preventing the electric current from flowing from the signal line 608 to a gate line 606 which is not selected (i.e., which is not supplied with a voltage).
FIG. 16 shows the thin film transistor 601 and the thin film light sensor 602 in more detail. The thin film transistor 601 and the thin film light sensor 602 are provided on one substrate 611 of the liquid crystal panel.
The thin film transistor 601 includes a gate electrode 612a, a gate insulating layer 613, a semiconductor layer 614, a highly doped layer 615, and a drain electrode 609. The gate electrode 612a is connected to the gate line 606. The drain electrode 609 and the source line 607 are connected to the semiconductor layer 614 via the highly doped layer 615. The highly doped layer 615 connected to the source line 607 acts as the source electrode of the thin film transistor 601. The drain electrode 609 is connected to the transparent pixel electrode. A light shielding film 616 is provided above a channel region of the semiconductor layer 614. The light shielding film 616 prevents malfunction caused by the semiconductor layer 614 being irradiated with external light.
The thin film transistor 602 includes a light shielding film 625, an insulating layer 626, a semiconductor layer 627, a highly doped layer 628, metal layers 623 and 629, and a gate electrode 612b. The semiconductor layer 627 is photoconductive. The gate electrode 612b is connected to the gate line 606.
The diode 603 includes a light shielding film 618, an insulating film 619, a semiconductor layer 620, a highly doped layer 621, and a metal layer 622. A light shielding film 624 is provided above the semiconductor layer 620. The metal layer 622 is connected to the signal line 608. The signal line 608 and the semiconductor layer 620 are connected to each other via the metal layer 622. The metal layer 623 and the semiconductor layer 620 are connected to each other via the highly doped layer 621. The diode 603 and the thin film light sensor 602 are connected to each other via the metal layer 623. The gate line 606 and the signal line 608 are connected to each other via the thin film light sensor 602 and the diode 603.
When a voltage is applied to the gate line 606, a voltage is applied to the gate electrode 612a of the thin film transistor 601 to form a carrier in the channel region in the semiconductor layer 614. When a voltage is applied to the source line 607 in this state, an electric current flows from the source line 607 to the drain electrode 609 via the semiconductor layer 614 to apply a voltage to the pixel electrode. By applying a voltage corresponding to image information to the gate lines 606 and the source lines 607 sequentially selected, a desired liquid crystal cell 660 is driven and thus an image is displayed.
When a voltage is applied to the gate line 606, a voltage is applied to the gate electrode 612b of the thin film light sensor 602. When the thin film light sensor 602 is irradiated with external light in this state, the semiconductor layer 627 becomes conductive. Thus, an electric current flows from the gate line 606 to the signal line 608 via the thin film light sensor 602 and the diode 603. More specifically, the electric current flows from the gate line 606 via the gate electrode 612b, the metal layer 629, the highly doped layer 628, the semiconductor layer 627, the highly doped layer 628, the metal layer 623, the highly doped layer 621, the semiconductor layer 620 and the metal layer 622 to the signal line 608.
The intensity of the light irradiating each thin film light sensor 602 connected to the gate line 606 supplied with a voltage is detected from the value of the electric current flowing in the corresponding signal line 608. By sequentially applying a voltage to the gate lines 606 to switch each pixel section 650 to an ON state or an OFF state and also detecting the value of the electric current flowing in the corresponding signal lines 608 sequentially, two-dimensional image information (color gradation pattern on the document sheet, etc.) is read.
Now, problems of the image input/output device 500 (FIG. 13) will be described.
The image input/output device 500 involves a problem that a sufficiently high resolution is not obtained for an image reading operation due to the low directivity of the light which is output from the light emitting element 502a. FIG. 17 shows an image reading operation of the image input/output device 500. Light which is output from the light emitting element 502a is reflected at point d on a printing face of the document sheet 503, and the reflected light is incident on the light receiving element 502b paring with the light emitting element 502a. In this case, the light travels on light path a. If only the light traveling on light path a is incident on the light receiving element 502b, the reflected light which is input to the light receiving element 502b has an intensity representing the color thickness of the printing only at point d. As a result, a sufficiently high resolution is obtained.
However, in general, the light which is output from the light emitting element 502a has low directivity and expands in other directions as well as traveling on light path a. For example, as shown in FIG. 18, the light output from the light emitting element 502a has a directivity characteristic of expanding widely with respect to the line vertical to the light emitting face. Therefore, as shown in FIG. 17, light traveling on light paths b and c and the like is also incident on the light receiving element 502b of interest in addition to the light traveling on light path a. The light paths b and c are for the light output from the pixel sections adjacent to the pixel section 502 of interest. The light receiving element 502b receives light reflected at points e and f on the document sheet 503 and traveling on light paths b and c together with the light reflected at point d and traveling on light path a. Namely, light reflected at a plurality of points on the document sheet 503 is incident on one light receiving element 502b at the same time.
When light reflected at a plurality of points on the document sheet 503 is incident on one light receiving element 502b at the same time, the color thickness information regarding one point on the document sheet 503 is also detected by a plurality of light receiving elements 502b located in the vicinity of the corresponding light receiving element 502b. Hence, the color thickness level of each point represented by the obtained color gradation information is blurred. Therefore, there arises the problem that a sufficiently high resolution is not obtained for the image reading operation.
Such a problem occurs also in the case of the display devices disclosed by patent documents 2 and 3 in a similar manner. FIG. 19 shows light which is output from the liquid crystal cell 660. Light output from a backlight 672 and reaching a liquid crystal panel 671 is transmitted through the liquid crystal cell 660 in an ON state and is output. When the directivity of the light output from the backlight 672 is low, light 675 output from the liquid crystal cell 660 in the ON state travels in directions close to a horizontal direction to the surface of the liquid crystal panel 672 as well as a direction vertical thereto. Even when the liquid crystal cell 660 is in an OFF state, the liquid crystal cell 660 outputs light 676 in directions close to the horizontal direction to the surface of the liquid crystal panel 671. Namely, whether the liquid crystal cell 660 is in an ON state or an OFF state, the liquid crystal cell 660 outputs light to directions close to the horizontal direction to the surface the liquid crystal panel 671. Therefore, light reflected at a plurality of points on the document sheet is incident on one light receiving element at the same time. As a result, there arises the problem that a sufficiently high resolution is not obtained for the image reading operation.
In order to solve such problems, an image display/input device disclosed by patent document 4 includes a directivity plate provided between a display/input liquid crystal panel and a backlight. The directivity plate receives light output from the backlight on an incident face and outputs the light after changing the traveling direction of the light to a direction vertical to the incident face. By supplying light having such a high directivity to the display/input liquid crystal panel, light traveling in directions close to a horizontal direction to the surface of the liquid crystal panel 671 is suppressed both in the ON state and the OFF state. Since light with a high directivity is always output from each pixel section, the resolution of the image which is obtained in the image reading operation can be prevented from decreasing.    Patent Document No. 1: Japanese Patent Publication for Opposition No. 5-40927    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 63-163886    Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 5-89230    Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 8-153179